1. Field of the Invention
The present invention relates to telecommunications. More particularly, the present invention relates to the transmission of data utilizing discrete multitone technology (DMT). The present invention is advantageously utilized in digital subscriber line (DSL) technology, although it is not limited thereto.
2. State of the Art
Recently, digital subscriber line (DSL) technology has been touted as the answer for the ever-increasing demand for transfer of information, and the requirement for higher and higher information transfer rates. DSL modems provide a much higher data rate than the convention V.34 and V.90-type modems. The DSL modems utilize discrete multitone (DMT) technology to transfer information. In DMT technology, a plurality of predefined frequencies (tones) are simultaneously subjected to quadrature amplitude modulation (QAM) in order to transfer information across a channel. In recently promulgated standards such as G.Lite and G.dmt standards (Recommendations G.992.1 and G.992.2 ITU-Telecommunication Standardization Sector, Study Group 15, MA-007 and MA-008, Melbourne Australia Mar. 29-Apr. 2, 1999) both of which are hereby incorporated by reference herein in their entireties, one hundred twenty-eight and two hundred fifty-six tones are specified respectively, with an integer number of bits of up to fifteen being transferred per tone. The actual tones utilized depends upon the signal-to-noise ratio (SNR) distribution of the channel. In particular, during a handshake sequence, the channel is scanned, and the SNR distribution and/or other parameters are measured.
The actual bit rate provided by a DMT-based system actually depends on the signal-to-noise ratio (SNR) distribution at the input of a receiver. The higher SNR per tone, the more bits the tone can carry (transfer). In turn, the SNR distribution is a function of signal attenuation and the noise power spectral density (PSD). It is well known that the signal attenuation is often a non-monotonic function of frequency, with one or more deep notches located along the frequency spectrum. In addition, PSD is not a flat function of frequency. As a result, the SNR is generally a multiextreme function of frequency.
An example of the SNR distribution for a 16 kft subscriber line is shown in Table 1. In this example, the first five tones would not be used as they would interfere with the "plain old telephone service" (POTS). The sixth tone has a SNR=29.51 dB. If the desired bit error rate (BER) is set equal to 10.sup.-7, the sixth tone can carry six bits, as a SNR=27 dB is required for transmission of six bits, while a SNR=30 dB is required for transmission of seven bits. The seventeenth tone, on the other hand, having a SNR=10.46 dB cannot even transmit a single bit, because a SNR of at least 11 dB is required to transmit one bit when the BER=10-.sup.7.
Some SNR adjustment is possible in DMT-based systems. For example, the transmitted level of the sixth tone in Table 1 may be increased by 0.49 dB to allow the tone to bear seven bits, with the transmitted level of the seventeenth tone may be increased by 0.54 dB to allow the tone to carry one bit. Thus, according to Section 11.12.14 and Section 11.11.13 of the G992.2 standard, and Sections 10.8.13 and 10.9.14 of the G992.1 standard, during initialization, the transmitting modem is provided information by the receiving modem regarding the number of bits to be sent (B) and the gain (G) for each tone being transmitted. However, the permissible signal gain is usually restricted. According to the previously incorporated G.lite standard, the maximum gain for any one tone is set equal to 2.5 dB. As a result, with a BER=10.sup.-7, no tone having a SNR&lt;8.5 dB can be used for data transmission. Using this criteria, it will be appreciated that in the case corresponding to Table 1, all tones with numbers 20 to 75 and 107 to 128 (as shown in bold type) cannot be used for data transmission. As a result, the actual bit rate is significantly reduced.